As semiconductor integrated circuit technology moves towards increasing device speed (Very High Speed Integrated Circuit, VHSIC) and towards the integration of ever larger numbers of functional elements as a single circuit (Very Large Scale Integration, VLSI) it is considered increasingly important to realize devices not at bulk semiconductor surfaces but in thin layers whose thickness is chosen to meet device specifications. Accordingly, methods are being sought for producing semiconductor crystals in the form of thin layers or films on an insulating substrate; desired layer thickness is typically less than approximately 2 micrometers, and desired layer diameter may be of the order of several inches.
Prominent among methods which have been proposed for the manufacture of such crystals are zone melting methods based on melting and resolidifying a layer of a starting or precursor material having a desired composition and being deposited on a dielectric, electrically insulating substrate; in this fashion device-grade layers may be obtained from amorphous or polycrystalline starting materials. For example, according to one method, melting and resolidifying take place as a laser beam sweeps back and forth while advancing in small steps; according to another, a graphite strip heater is moved slowly over a layer to be crystallized.
For the case of silicon films, the former technique is described, e.g., in the following papers:
J. F. Gibbons, "CW Laser-Recrystallized, Polysilicon as a Device-Worthy Material", Laser and Electron-Beam Solid Interactions and Materials Processing J. Gibbons et al., editors, North Holland, 1982, pp. 449-462;
L. E. Trimble et al., "Seeded Growth of Si over SiO.sub.2 Substrates by CW Laser Irradiation", Laser and Electron-Beam Interactions with Solids B. R. Appleton et al., editors, North Holland, 1982, pp. 505-510; and
C. K. Celler et al., "Seeded Oscillatory Growth of Si over SiO.sub.2 by CW Laser Irradiation", Applied Physics Letters, Vol. 40 (1982), pp. 1043-1045.
Similarly, the use of a graphite strip heater is described, e.g., in the following papers:
M. W. Geis et al., "Zone-Melting Recrystallization of Encapsulated Silicon Films on SiO.sub.2 -Morphology and Crystallography", Applied Physics Letters, Vol. 40 (1982), pp. 158-160;
R. F. Pinizzotto et al., "Subgrain Boundaries in Laterally Seeded Silicon-on-Oxide Formed by Graphite Strip Heater Recrystallization", Applied Physics Letters, Vol. 40 (1982), pp. 388-390; and
J. R. Lineback, "Oxide Insulator Looks the Equal of Sapphire for C-MOS ICs", Electronics, June 2, 1982, pp. 45-46.
Methods as described in the above-cited references demonstrate the applicability of zone melting for the production of thin-film semiconductor crystals. However, while considered satisfactory for producing small-diameter crystal layers, these methods are considered not to be readily adaptable to the production of large-size, commercially significant crystal films.